This invention relates to a method for producing blow-moulded containers.
Many millions of containers are produced annually around the world by blow moulding. Blow moulding involves extruding a polymer composition through a die to form a discrete, hollow extrudate, inflating that extrudate against the mould surface to form the container, and removing the container from the mould surface. The polymer compositions which may be used for producing blow-moulded containers include olefin polymers, olefin co-polymers and vinyl chloride polymers.
Blow-moulded containers have been produced which contain a small amount of particulate filler such as calcium carbonate. To the best of the Applicant's knowledge, no blow-moulded containers containing more than 15% by mass of a particulate filler have been produced on a commercial scale. The reason for this is that if more than 15% by mass of inorganic filler is added to the polymer composition, the container which is produced has unacceptable physical, mechanical and other properties. In particular, it has been found that the impact resistance, an important characteristic of most blow-moulded containers, is sharply reduced with the higher contents of inorganic filler. There is thus a prejudice in the art against using more than 15% by mass of inorganic filler in blow-moulded containers.
Particulate fillers are used for manufacturing other articles from polymer compositions. For example, in injection moulding, relatively high contents of inorganic filler can be tolerated without adversely affecting the properties of the article. The compositions may also include a coupling agent such as that described in British Patent Specifications Nos. 1,525,418, 1,509,283 and 1,515,645. It is to be noted that the compositions described in the three British patent specifications are all useful for injection moulding. There is no suggestion teaching in any one of these specifications that the combination of a particulate filler and a coupling agent would be useful in polymer compositions useful for producing blow-moulded containers.
British Patent Specification No. 905,069 describes a polymer composition comprising a blendable polyolefin and a finely-divided coated carbonate filler. The coating of the carbonate filler may be a higher fatty acid. The specification suggests that the compositions may be useful for moulding, blowing, extruding and pressing. There is no suggestion that the compositions may be useful for producing blow-moulded articles and indeed, there is no working example of the fabrication of the composition into any particular shaped article.
Japanese Patent Publication No. 53-132,049 describes a polymer composition containing an inorganic filler and dibenzylidenesorbitol. The composition is said to produce shaped articles which have improved surface lustre. It is believed that the surface lustre is produced by the dibenzylidenesorbitol, a hydrophilic and organophobic material, migrating through the polymer to the surface of the shaped article. The specification states that the composition may be moulded by calendar moulding, extrusion moulding, injection moulding and blow moulding to produce films, sheets and hollow bottles. It is to be noted that there is no working example illustrating the production of hollow bottles by blow moulding the compositions. The melt indices of the polymers used in the working examples indicates that in each example an injection moulded article was produced.
The patent literature also contains other publications describing polymer compositions containing inorganic fillers and coupling agents. However, none of this art describes or suggests that such compositions would be useful in producing blow-moulded containers.